In an orthopedic surgical procedure which requires the cutting or shaping of bone, achieving higher accuracy in cutting or shaping the bone can yield superior surgical outcomes. To this end, orthopedic surgical robots such as the ROBODOC® surgical assistant system (Curexo Technology Corp., Fremont, Calif., USA) have been developed. These orthopedic surgical robots utilize computer-controlled surgical cutters which allow superior accuracy to be achieved in the cutting or shaping of bone.
With the current ROBODOC® system, the surgeon uses, prior to surgery, pre-operative surgical planning software to plan the intended position of the implant vis-à-vis the bone. More particularly, CT scan data is processed so as to provide a virtual model of the bone, an implant library is provided which comprises virtual models of the various implants available to the surgeon, and the pre-operative surgical planning software allows the virtual model of the implant to be merged with the virtual model of the bone so as to enable the surgeon to visualize the final implant position vis-à-vis the bone, and so as to enable the surgeon to see the tool path which will be used by the surgical robot to cut the bone so that the bone can properly receive the implant.
By way of example but not limitation, U.S. Pat. No. 5,824,085 discloses a system which may be used for pre-operative surgical planning. This system accepts a virtual model of the bone which is to receive the implant. This virtual model of the bone is typically generated using CT scan data of the bone. The system also stores a library of virtual models of the implants which are available to the surgeon. Using the pre-operative surgical planning system, the surgeon can superimpose the virtual model of the implant on the virtual model of the bone so as to visualize, prior to the actual surgery, the final position of the actual implant vis-à-vis the actual bone. The pre-operative surgical planning system also shows the tool path which will be used by the surgical robot to cut the bone to receive that implant. Thereafter, after the surgical robot and the bone have been placed into proper registration with one another, the surgical robot can automatically cut the bone according to the planned data, so that the bone can properly receive the implant.
However, the bone which is to receive the implant is typically surrounded by soft tissue such as muscles, tendons, blood vessels, nerves, skin, etc. During the orthopaedic surgery, care should be taken to preserve those soft tissues. Among other things, and of special concern with respect to the present invention, during the cutting of the bone, the cutting tool (e.g., the saw blade or high-speed rotating cutter) should not damage the soft tissues located beyond the bone, i.e., by penetrating beyond the bone. By way of example but not limitation, in total hip replacement surgery and/or in total knee replacement surgery, it is important to avoid damaging soft tissue structures (e.g., delicate neurovascular structures) which lie adjacent to the bones being cut.
To prevent such soft tissue damage during the cutting of bone, several approaches has been used in manual surgical procedures. For example, a curved metal protector is sometimes slid between the bone and the soft tissue so as to protect the soft tissue from the cutting tool. Or, a “plunging cutter” (or drill) can be equipped with a physical stop mechanism or depth gauge so that the surgeon does not inadvertently cut beyond the boundary of the bone.
Using a robotically-controlled cutting tool, it is possible to achieve precise cutting accuracy when cutting the bone. In order to achieve optimum cutting accuracy with minimized cutting time, it is desirable for the tool path, and the feed rate of the cutting tool, to be pre-programmed. At the same time, however, inasmuch as each bone has a unique shape for each patient, the pre-programmed tool path should be customized for each patient. By way of example but not limitation, if the pre-programmed tool path is not customized for each patient, there is the possibility that the bone may be overcut. More particularly, if the planned implant volume extends outside the bone, then the surgical robot's cutter moves beyond the outer boundary of the bone and will engage any anatomical structures located within the planned implant volume. Thus, the cutter can cause damage to soft tissue (or other anatomical structures) surrounding the bone. See, for example, FIGS. 1A-1D, which are schematic views showing a tibial implant (FIGS. 1A and 1B), the pre-defined tool path for the tibial implant, which requires a surgical opening of width L (FIG. 1C), and possible soft tissue damage where the implant is larger than the patient's bone (FIG. 1D).
Moreover, inasmuch as the surgical opening used during the operation may vary in size, shape and/or location, the direction of approach of the tool path should take into account the nature of the surgical opening so that movement of the surgical robot does not collide with the perimeter of the surgical opening. Also, as mentioned above, the robotic cutting should ensure that the cutting does not damage soft tissue adjacent to the bone. At the same time, the use of conventional metal protectors, which is common in manual surgery, is problematic in robotic surgery because, if the cutter collides with the metal protector, the cutter and protector may be damaged and may leave metal debris in the patient's body. Furthermore, during typical robotic surgery, there is generally insufficient room to accommodate a metal protector due to the presence of the robot tool or robot arm.
Thus, there is a need for a new method and apparatus for generating an improved tool path for an implant which minimizes soft tissue trauma, e.g., by utilizing a pre-determined tool path but also protecting the soft tissues behind or around the bone, and also minimizing the required surgical access.